GB2121812A - Sealing coating for a hydrophilic colloid layer - Google Patents

Abstract

A coating composition suitable for applying as a sealing coating to a hydrophilic colloid, comprises a mixture of at least one polyurethane resin and at least one polyisocyanate precursor compound, characterized in that 1) the said polyurethane resin is obtained by the reaction of one or more polyisocyanates and one or more polyols in a proportion such that substantially no free isocyanate groups are present in the resulting polyurethane, and 2) the said polyisocyanate precursor compound is the addition product of a polyisocyanate with at least one compound selected from the group acetone-oxime, butanone-oxime, phenol, monochlorophenol, a di(C1-C2) alkylmalonate, the said mixture consisting of from 60 % to 95 % by weight of the said polyurethane resin and from 5% to 40 % by weight of the said polyisocyanate precursor compound(s). Also disclosed is a process for forming a laminar article in which a web or sheet material having an outer hydrophilic colloid layer is contacted with a sealing coating, as defined above, applied on a web or sheet support, and then heating to free polyisocyanate in the coating composition, whereby the coating becomes sealed to the colloid layer by a cross-linking reaction with the colloid. The laminar article may be produced by sealing said supported sealing coating to a photograph containing a layer including a silver image and/or colour image in a proteinaceous hydrophilic colloid binder. Uses:-Indentification documents.

Description

SPECIFICATION Sealing coating for a hydrophilic layer This invention relatesto a coating composition suitable forapplying as a sealing coating to a hydrophilic colloid layer and to a process for producing a laminararticle.

This production of laminar articles comprising a protective cover sheet or layer on an information bearing surface e.g. of a photograph is well known.

For example, identification documents contain a black-and-whiteorcolourphotograph sandwiched between a clear plastic cover sheet and a rear support sheet. The assembly is laminated together to provide a durable identification document also called ID card.

In view of the widespread use of ID cards as security document e.g. to establish a person's authorization to conduct certain activities (e.g. driver's licence) orto have accessto certain areas orengage in commercial actions, it is importantthatforgery ofthe ID card by alteration of certain of its data and/or photograph is made impossible. Adhesive systems have been proposed to provide a security seal through a lamination technique.In case such a seal exists an attemptto pull the protective plastic sheetfrom the layer containing the information results in a destruction of the layer containing the information or the protective sheet adheres so firmly to the image layer that it cannot be pulled away without removing the image layerfrom the image receiving element as a whole.

Astill better protection would be obtained when the securityseal formed by an adhering coverwould also notallowthe removal oftheplasticcoverby means such as melting and hot water or organic solventtreatmentofthe laminararticle.

In the published Japanese PatentApplication No.

75/93429 of Fuji Photo Film Company published July 25,1975 a method forthe protection of photographs is described wherein to the photograph a resin coating containing an isocyanate or an isocyanate precursor is applied and said coating itself is carried by a poly(vinylchloride) support serving as protective cover sheet. The sealing to the photograph is effected by heating the resin coating at 100-120 Cwith hot rollers applying a pressure of 3 kg/sq.cm to the materials to be sealed.

According to example 1 of said published Japanese Patent Application a poly(vinylch bride) sheet of a thickness of 120 m is coated with a resin coating of 40 cm containing a mixture of 50 parts by weight of a polyurethane and 50 parts by weight ofthe addition product of 1 mole oftrimethylolpropane and 3 mole oftolylene diisocyanate. The polyurethane has been obtained by reaction of 1 mole of polyethylene oxide with an average polymerisation degree of about 12 and 2 moles of tolylene diisocyanate which means that the polyurethane contains a considerable amount of free isocyanate groups.

According to example 3 of the same document, the isocyanate groups of the addition product of example 1 are temporarily blocked by reaction with 3 mole equivalentofethanolwherebyaso-called isocyanate precursor is formed yielding free isocyanate groups by heating on splitting off the ethanol.

The above protective coating compositions yields an excellent sealing by reaction with a gelatincontaining coating of a photograph but only immediately, or within a small period of a few hours, after preparation ofthe coating compositions. This is because they self-harden relatively easily probably due to reaction of free isocyanate groups with water of the atmosphere forming thereby on crosslinking ureido-g roups through the intermediate formation of amines which form substituted urea, as described by D. H. Solomon in his book "The Chemistry of Organic Film Formers"-Johnwiley & ons,Inc., NewYork (1967) p.202. As a result ofthis self-hardening reaction the sealing coatings of the above exemplified compositions lose their plasticity after a few hours at room temperature (20 C), and thus cannot be coated on by the application of heat.

It is an object ofthe present invention to provide a supported sealing coating with improved storage properties suitable forthe sealing of hydrophilic colloid coatings, e.g. of photographs.

It is another object of the present invention to produce with said supported sealing coating a laminate e.g. an identification card protected against forgery.

Other objects and advantages will become apparentfromthefurtherdescription.

In accordance with the present invention a coating composition suitableforapplying asa sealing coating to a hydrophilic colloid layer is provided, said composition comprising a mixture of at least one polyurethane resin and at least one polyisocyanate precursor compound, characterized in that 1) the said polyurethane resin is obtained by the reaction of one or more polyisocyanates and one or more polyols in a proportion such that substantially no free isocyanate groups are present in the resulting polyurethane, and 2) the said polyisocyanate precursor compound is the addition product of a polyisocyanate with at least one compound selected from the group acetone-oxime, butane-oxime, phenol, monoch lorophenol, a di(C1-C2) alkylmalonate, the said mixture consisting offrom 60% to 95% by weight of the said polyurethane resin and from 5% to 40% by weight of the said polyisocyanate precur sor compound(s).

A polyurethane resin containing substantially no free isocyanate groups may be obtained by the reaction of the said polyisocyanate(s) and the said polyol(s) in proportions such that the ratio ofthe totals of-OH groups and-NCO groups in the reactants is in the range of 1.05:1 to 0.95:1; preferably said ratio is 1:1.

In said composition the polyol(s) used in the preparation ofthe said polyurethane resin is (are) preferably an aliphatic or cycloaliphatic diol, or a polyoxyalkylenediol ora mixture thereof. Examples of these diols also called glycols are: ethylene glycol, 1,2- and 1,3-propylene glycol, 1.4- and 2,3-butylene glycol, 1,6-hexane diol, neopentyl glycol, cyclohexane dimethanol, diethylene glycol,triethylene glycol, tetraethylene glycol, polyethylene glycol, dipropylene glycol, polypropylene glycols, dibutylene glycols, polybutylene glycols and 1,2-cyclohexanediol or are polyoxyalkylene diols e.g. polyoxyethylene diols or polyoxyethylene glycols according to the followingformula:: H-(OCHrCH2)n-OH wherein n is a positive integerfrom 5 to 25.

In the production of the polyurethane compound (1) more preferably a mixture of an alkylene diol and a polyoxyalkylene diol is used which mixture contains at least 68-75 mole % of a polyoxyethylene diol having preferablyfrom 8 to 9 recurring oxyethylene groups.

Suitable polyisocyanate compoundsforforming the polyurethane (1) are aliphatic,cycloaliphaticor aromatic diisocyanates. Examples thereof are 1,6hexane diisocyanate, 1 ,4-cyclohexanediisocyanate, 4,4'-diisocyanate-dicyclohexylmethane, 1 ,4-pheny- lene diisocyanate, 2,6-tolylene diisocyanate, 4,4'diisocyanato-diphenylmethane and isophoron diisocyanate having the following structural formula::

In a particularly useful embodimentofthe present invention the sealing coating composition contains a polyurethane (1) which is the addition reaction product of a mixture of diols and a mixture of diisocyanates in equimolar amounts, the said mixture of diols consisting of 4-30 mol % of ethylene glycol (HO~CHrCH2~OH) and 60-70 mol % of a polyoxyethylene glycol [ HO-(CH2-CHT-O)Fi-OH ] wherein n is 8to 9 and the said mixture of diisocyanates consisting of a 25/75 to 50/50 molar mixture of isophoron diisocyanate and 4,4'-diisocyanato-diphenylmethane.

The use of a higher amount of ethylene glycol in the production ofthe addition product reduces the stickiness and adhesiveness of the sealing coating and yields less good results on sealing.

The hydroxyl value of the latter polyurethane compound is preferably not higher than 56 so that its molecularweight is at least 2000. The hydroxyl value (according to British Standard 684:190) is the number of mg of KOH required to neutralize the acetic acid capable of acetylating 1 g of the compound involved.

A polyurethane ofthe latter composition preferably has a molecularweightso high that the viscosity of a 20% by weight solution thereof at 250C intetrahydro- furan is from 1 Oto 90 m.Pa.s. At lower viscosity valuesthesolidityofthesealing layer diminishes rapidly and at higherviscosityvaluesthe sealing needs an extra heating.

Preferred polyisocyanate precursors (2) correspondto one ofthefollowing structures: P1, P2 orP3

wherein R stems from a low molecular weight ( < 2000) aliphatic polyurethane.

The latter polyisocyanate precursor(s) P1, P2 and P3 have a good stability at room temperature (20 C) and regenerate efficiently the free isocyanate under the conditions of heat-sealing above 100#C. A precursor obtained by blocking free isocyanate groups with ethanol has been described e.g. in Example 3 of the already mentioned JA-PatentAplication but said precursor proved to be too stable and therefore useless in the present combination with polyurethane (1).

By saying thatthe present composition forforming a sealing coating essentially consists of a mixture of at least one polyurethane resin 1) and a polyisocyanateprecursor2) is meantthatadditional components in amounts which do not substantially alter the sealing properties may be present e.g. dyestuffs, pigments, ultraviolet-radiation absorbing compounds and matting agents.

The preparation of a protective material suited for use in the production of a laminar article according to the present invention proceeds by applying a mixture of compounds (1) and (2) from a solution in a suitable organic solvent to a web or sheet material serving as a support e.g. a subbed or unsubbed hydrophobic transparent resin support and subsequently removing the solvent by evaporation to form a sealing coating having the above composition taking care that the temperature of the protective material does not exceed 800C e.g. by using an air stream of 11 00C striking overthe layer for 2 minutes. The evaporation ofthesolventskeepsthetemperature below80'C.

According to a technique allowing problem-free storage of a sticky protective material a siliconetreated paper sheet or web is used as an interlying strippable material preventing direct contact of the sealing coating with its backing in stacked or rolledup state.

The thickness of the sealing coating is preferably in the range of 5 to 100,am and preferably in the range of 10 to 30,am in otherwords the coating proceeds preferably at a dry coverage of 10 g/sq.m to 30 g/sq.m.

The protective material may be produced in the form of a web or sheet.

In the present process for forming a laminararticle a web or sheet material having an outer hydrophilic colloid layer is contacted with said sealing coating applied on a web or sheet support and thereafter heated to free polyisocyanate in the coating composition whereby the coating becomes sealed to the said colloid layer by a cross-linking reaction with the hydrophilic colloid.

During the heating process isocyanate groups in the blocked polyisocyanate (2) are set free again.

These free isocyanate groups play a double role in that a part of them makes crosslinks with the urethane groups of the elastomer (1) by forming allophanate groups and another partofthem reacts with said active hydrogen atoms e.g. ofthe HO- or NHrgrnups of a proteinaceous colloid such as gelatin to form therewith urethane and ureido groups respectively.

A particular useful procedure for preparing the laminar article is to place the sheet material carrying a hydrophiliccolloid layer to be protected in superposition with the sealing layer of the protective material and then to pass the superposed elements through the nip of a pair of hot pressure rollers. A heating in the temperature range of 1 to 1300Cfor a period of time lasting preferably not longer than 30 s is sufficientto obtain a chemical bonding of a hydrophilic colloid layer containing reactive hydrogen atoms such as gelatin with the present heat-crosslinkable sealing layer.By said process a laminarstructureis obtained in which the chemically bonded layers are no longerseparatable by tearing, melting and hotwater or organic solventtreatment.

According to a particular embodiment the sealing layer is pre-heated for a short period e.g. 1 to 5 minutes before effecting the lamination step in order to activate the sealing.

In a preferred embodimenttheformation of a strongly crosslinked structure may proceed byfirst passing the protective material with its sealing layer in contact with the sheet or web material having a supported gelatin containing layer to be protected through the nip of a pair of heated (up to 120 C) pressure exerting rollers and secondly by post heating the laminate without pressure for a period of afewmin e.g.5 min inthe rangeof 100to 110 C.The applied pressure by the rollers may be in the range of Sto 0.1 kg/sq.cm. The laminate provided by said embodiment has been found to be so strong asto resist completely all attempts of separation of the laminated sheets.The crosslinked structure of the elements ofthe seal makes it practically impossible to penetrate with a razor blade and to separate the hydrophilic colloid layerfrom the crosslinked sealing layer.

In the protection of photographs the present sealing coating is preferably applied to a transparent hydrophobic resin supported. asubbedstronglytear resistant polyester support e.g. a polyethylene terephthalate resin support. Such support may have a thickness in the range of 0.1 to 0.05 mm. Suitable subbed polyester resin supports are described e.g. in GB-P 1,234,755.

Other suitable supports are polyethylene coated paper and polyvinyl chloride supportsthat may contain a matting agent.

Hydrophilic colloids containing reactive hydrogen atoms with respectto free isocyanate groups are well known to those skilled in the art. Such hydrophilic colloids include proteinaceous colloids such as gelatin, hydrophilic cellulose ethers and esters containing free hydroxyl groups e.g. partially hydrolyzed polyvinyl acetates, polyvinyl alcohol, alginic acid and polyvinyl pyrrolidone.

Gelatin including partly hardened gelatin isthe most commonly applied binder in image-recording materials such as photographic silver halide emulsion layer materials and image-receiving materials applied in a diffusion transfer reversal process.

A particularly useful application ofthe protective material according to the present invention is therefore in the protection of photographs containing a layer including a silver image and/or colour image in a proteinaceous hydrophilic colloid binder by sealing thereto said protective material through its sealing layer.

According to an embodimentthe silver image is formed by means of a physical development of diffusion-transferredsilvercomplexcompounds under the catalytic influence of developing nuclei in an image-receiving layer. According to several other embodiments diffusion transfer processing may likewise result in an image-wise distribution of a diffusible dye in a therefor adapted hydrophilic colloid receiving layer.

For the latter processing and materials used therein reference is made e.g. to The Theory of the Photographic Process by T. H. James, 4th ed., Macmillan Publishing Co., Inc. New York (1977)366-372 and the US Patent 4,232,1 07.

The hydrophiliccolloid layer to be protected can be coated on a sealing layer as described above and then another sealing layerwith said compounds (1) and (2) can be laminated thereto withoutthe above mentioned post-heating. Such procedure is particularly interesting forthe production of identification cards (ID cards) with an excellent security seal.

In an embodimentofthatprocedureasubbed polyester or polyvinylchloride support is first coated with the above described sealing coating composition e.g. at a thickness of about 5,mm and dried for a few minutes at 60 C. Thereupon according to a well known procedure e.g. described in US-P 4,242,436 layer, a developing nuclei containing layerincorporating these nuclei in gelatin as binding agent is applied to the dried sealing coating.The thus obtained diffusion transfer reversal (DTR-) image receiving material is used to produce thereon a silver image which is then protected against contact with the environment and againstforgery by laminating thereto a protective material according to the present invention e.g. by passing the sandwiched materials between the nip of heated (100-130 C) metal rollers exerting a pressuretothesandwiched materials in the range of 0.1 to 0.5 kg per sq.cm operating with a transport speed such that each element is pressed togetherfor 15 to 20 s.

Said procedure is not restricted to the use of recording material having a polyester resin or polyvinylchloride support. Any support that does not stain on said heating and does not show detrimental distortions may be used.

According to a special embodiment a recording material comprising visually detectable information and containing one ortwo outer hydrophilic colloid layers with reactive hydrogen atoms is encased between two protective sheet materials ofthe present invention one form ing a transparent front wall memberand the other a transparent or opaque rear wall memberofthe lamina r article, the front and back members are then preferably somewhat larger in dimensionsthan the information comprising mate rial,sothatby heat and pressure lamination ofthe protective sheet materials with the encased material a peripheral area of the same chemical crosslinked structure of superposed sealing coating edges makes that a very tight seal is formed.The protective rear member, when transparent, allows the presence of visually detectable information on the rear side of the support carrying already information on the front side.

In this connection referenceismadetotheproce- duredescribed in the United Kingdom Patent Application 8130557 relating to the production of a sealed laminar article comprising a support sheet, an image layer containing a photographic image contained in a hydrophilic colloid medium and a protective sheet and in which said procedure proceeds by the steps of 1) providing a support sheetformed of a polymeric heat meltable material, 2) disposing adherently said image layer, before or after image formation therein, on the support sheetto coverthe support sheet only partlyand to provide at least one edge area of said support sheet to be free of said image layer, 3) providing a protective sheetformed of a heat meltable polymeric material which when in the melted state is capable of forming a homogeneous mass with melted support sheet material, 4) heat sealing said protective sheetto said support sheet in the area of the support free of said image layer to provide said homogeneous mass, where- by said image layer is sealed between said support sheet and said protective sheet, prevent ing thereby peeling apart of said support sheet and protective sheet material.

Said support sheet is e.g. a heat meltable hyd rophobicvinylchloride homopolymer or copolymer.

In an embodiment ofthe present invention derived from said procedure a recording material containing a pattern-wise applied image layer is sealed to said protective sheet by the present sealing coating wherein the sealing coating is applied over an area substantially the same or differentfrom the area of said image layer.

By applying the chemically reactable sealing coating having a composition as described herein in a pattern different from the pattern formed by the hydrophilic colloid image layer e.g. in the form of a line or chess-board pattern overthe image layer a local destruction ofthe image layer may result by forgery. Such local destruction may be easily detected by the eye.

In a preferred system for forming an ID card a diffusion transfer reversal process is applied to form photographically a portrait and descriptive information simultaneously on one andthe same support carrying a hydrophilic colloid image receiving layer.

Acamera suited for said process is described in the USP4,01 1,570 which camera operates with a photosensitive silver halide emulsion material used in conjunction with a separate image-receiving material whereon through the well-known silver complex diffusion transfer reversal (DTR-) process a positive silver image (portrait and descriptive matter image) is formed.

The information applied atthe rearside may be descriptive matter intheformofindicia(e.g.

numbers and letters) e.g. a signature or finger print.

The ID card may contain likewise identifying matter in the form of magnetizable, fluorescent or ultraviolet or infra-red light absorbing matter which is not detectable visually. Such matter is preferably applied priorto sealing and protected bythe seal. The identifying matter may be supplied likewise in the form of embossed material parts or as holes applied in a coded pattern which pattern can be recognized by known devices.

Other specific information which may be included in the present laminararticle is e.g. in the form of a dye spot having a specific absorption spectrum recognizable by an electronic photocell detection apparatus. The laminar article obtained according to the present invention may include all kinds of compounds playing a role in the stability of photographic image information e.g. ultra-violet light absorbers and anti-oxidants. Duetothe high moisture-resistance of the seal obtained according to the present invention no degradative effects resulting e.g. from hydrolysis and bacterial influences have to be feared.

According to a special embodiment within the scope of the present invention the protection of a hydrophiliccolloid layer proceeds by the present coating composition by applying said composition from a solution ofthe said polyurethane resin 1) and the said polyisocyanate precursor 2) onto the hydrophilic colloid layer to be protected, whereupon the solvents) of the solution are removed by evaporation.

The sealing is obtained on heating by the already described crosslinking reaction with freed active polyisocyanate.

In the following Example the use of coating compositions according to the invention in comparison to prior art compositions with regard to the protection of information contain in an image receiving materialforthe silver complex diffusion transfer reversal (DTR-) processing is illustrated.

All parts, percentages and ratios are by weight unless otherwise indicated.

Example Production of protective material I The homogeneous mixture of the following ingredients A and B was coated onto a double side subbed polyethyleneterephthalate support of 100,am to form at a wet coating thickness of 150 jum a dry sealing layer of 30 ym.

A. 20 ml of a 20% solution in methylene chloride of a polyisocyanate precursor P1 according to the structure given hereinbefore.

The preparation of precursor P1 is described furtheron.

B. 80 ml of a 20% solution in tetrahydrofuran of a polyu rethane elastomer E1 of ethylene glycol (1), poly(oxyethylene) glycol (2) (average molecular weight400) and isophorondiisocyanate (3) and 4,4'-diisocyanatodiphenylmethane (4).

The molar ratio of (1) to (2) is 32.5/67.5.

The molar ratio of (3) to (4) is 25/75.

The molar ratio of (1) + (2) to (3) + (4) is 1 so that no excess free isocyanate groups are available.

The preparation of the elastomer El is described furtheron.

The sealing coating was dried for 2 minutes in an air stream of 90 C.

Production of protective material II Material II was prepared as material I with the difference, however,thatthepolyurethaneelastomer E1 was replaced by the polyurethane elastomer E2the preparation of which is given furtheron.

Production of protective material Ill Material Ill was prepared as material I with the difference, however, that the polyurethane elastomer E1 was replaced by the polyurethane elastomer E2 and the polyisocyanate precursor P1 was replaced by the polyisocyanate precursor P2 the preparation of which is described furtheron.

Production of protective material IV Material IV was prepared as material I within the difference, however,thatthe polyurethaneelastomer E' was replaced bythe polyurethaneelastomerE2 and the polyisocyanate precursor P1 was replaced by the polyisocyanate precursor P2 the preparation of which is described furtheron.

Production of the protective material V Material V was prepared as material I with the difference, however,thatthe polyurethaneelastomer El was replaced by polyurethane elastomer E2 and 20 parts by weight of polyisocyanate precursor P1 were replaced by a mixture of 10 parts by weight of polyisocyanate precursor P1 and 10 parts by weight of polyisocyanate precursor P4 the preparation of which is given furtheron.

Production of the protective material Vl Material Vl was prepared as material I with the difference, however, that the polyurethane elastomer E1 was replaced by polyurethane elastomer E3 still containing a substantial amount of free isocyanate groups and the preparation of which is given furtheron and the polyisocyanate precursor P' was replaced by the polyisocyanate Psthe preparation of which is given furtheron, the elastomer E3 and the polyisocyanate P5 being used in a 50:50 ratio.

Production of protective material VII Material VII was prepared as material I with the difference, however, that the polyurethane elastomer elastomer E' was replaced by the polyurethane elastomer E3.

Production of protective material VIII Material VIII was prepared as material I with the difference, however, that the polyurethane elastomer E1 was replaced by the polyurethane elastomer E3 and the polyisocyanate precursor P' was replaced by the polyurethane precursor P3.

Production of protective material IX Material IX was prepared as material I with the difference, however, that the polyurethane elastomer E1 was replaced by the polyurethane elastomer E3 and the polyisocyanate precursor Pa was replaced by polyisocyanate precursor P2.

Production of protective material X Material X was prepared as material I with the difference, however, that the polyurethane elastomer E1 was replaced bythe polyurethane elastomer E4.

The protective materials I to Ill and V are according to the present invention, the protective materials IV andVltoXarenot.

Production ofa laminated identification card(lD-card) with saidprotective materials I to X A processed DTR-silver image material having a silver image contained in an image receiving gelatin layer applied on a subbed polyethyleneterephthalate support is conveyed with the gelatin layer containing the silver image in contact with the sealing layer of the protective material through the nip of a roller laminatorofwhich the steel rollers have a temperature of 1 300C and are coated with silicone rubber.

Hereby the temperature of the sealing layer reached a value of about 1 l00Cwhichwas maintained for about 12s.The pressure between the rollers was 0.25 kg/sq.cm.

Evaluation ofthe sealing properties ofthe above described protective materials I to Xin the protection ofa silver image present in said ID-card The evaluation proceeded: A: practically immediately after coating and drying the sealing layer, B: after keeping the sealing layer of the protective material under normal atmospheric conditions (20 C, relative humidity of 60%)for 1 week without liquid post-treatment C: A + a post-treatment for 24 h in a waterbath at 55 C, D: B + a post-treatment for 24 h in methylene chloride (a solventforthe layer composition before the cross-linking reaction).In the evalua tionthefollowing quotation numbers were given: 1: a peel apartattemptdoesnotsucceedandthe silver image by treating the laminate with a 10% aqueous FeCl3solution is not affected (does not bleach).

2: a peel apart attempt succeeds but the treatment with said FeCl3solution does not bleaches the silver which means that the sea ling layer still adheres substantiallyto the gelatin layer and forms an impermeable protective coating thereon.

3: a peel apart attempt succeeds and the FeCI3 solution treatment destroys partlythe silver image.

4: a peel apart attempt succeeds and the the FeCI3 solution treatment destroys completelythe silver image.

The evaluation results are listed in thefollowing Tables 1 and 2.

Table 1 Protective Keeping Stickiness Without After the After the material time of the liquid water CH2C12 after sealing post- treatment treatment drying layer treatment I 5 min not sticky 1 1-2 2 1 week " 1 1-2 2 II 5 min not sticky 1 1-2 2 1 week " 1 1-2 2 III 5 min not sticky 1 1-2 2-3 1 week " 1 1-2 2-3 IV 5 min not sticky 4 - - 1 week " 4 V 5 min slightly 1 3 3 sticky 1 week " 1 3 3 VI 5 min very 1 2-3 2-3 sticky 1 week not sticky 4 - VII 5 min very 1 2-3 2-3 sticky 1 week not sticky 4 - VIII 5 min very 1 3 3 sticky 1 week not sticky 4 - IX S min very 1 3 3 sticky 1 week not sticky 4 - X 5 min very 1 3 3 sticky 1 week not sticky 4 - - : no adherence anymore An extra heat-treatment ofthe laminated articles of Table 1 by keeping them for 5 minutes at 1 1 OOC yieldedthefollowing evaluation results listed in Table 2.

Table 2 Protective Keeping time Without After the After the material after drying liquid water CH2C12 post-treatment treatment treatment I 5 minutes 1 1 2 1 week 1 1 2 II 5 minutes 1 1 2 1 week 1 1 2 III 5 minutes 1 1-2 2-3 1 week 1 1-2 2-3 IV 5 minutes 4 - 1 week 4 - V 5 minutes 1 2-3 2-3 1 week 1 2-3 2-3 VI 5 minutes 1 2-3 2-3 1 week 4 - VII 5 minutes 1 2 2 1 week 4 - VIII 5 minutes 1 2-3 2-3 1 week 4 - IX 5 minutes 1 2 2-3 1 week 4 - X 5 minutes 1 2 2 1 week 4 - Preparation of polyurethane elastomerE7 in a reaction flask of 250 ml provided with a thermometer, stirrer, dropping funnel and reflux condenser having atthetop acalcium chloride drying tube 37.2 g (0.15 mol) of 4,4'-diisocyanato-dipheny Imethane and 11.1 g (0.05 mol) of isophoron diisocyanate were dissolved in 60 ml of anhydrous tetrahydrofuran.

To the solution, cooled at 90C in an ice/waterbath a solution of 54.0 g (0.135 mol) of anhydrous polyoxyethylene glycol (average molecularweight400) in 20 ml ofanhydroustetrahydrofuran were added dropwise over a period of 2.5 h.

The reaction proceeded exothermally and the temperature rised from 9 Cto 230C and the reaction mixture became viscous.

Thereupon a solution of 4.03 g (0.065 mol) of ethylene glycol dissolved in 20 mtofanhydrous tetrahydrofuran were added. Said addition did not substantially raise the temperature and the ice/ waterbath was removed.

Subsequently the reaction mixture was kept still ring for 18.5 h at 25-26 C.

The very viscous solution was diluted with tetrahydrofuran upto 503 ml.

The solution contained 20 g of elastomer E1 per 100 ml and was used as such.

At 250C the viscosity of a 20% solution in tetrahydrofuran was 20.3 mPa.s.

By infra-red analysis no free isocyanate groups could be detected.

Preparation of polyurethane elastomerE2 In a reaction flask of 11 provided with a thermometer, nitrogen gas inlet, a stirrer, dropping funnel and reflux condenser having at the top a calcium chloride drying tube, 54 g (0.135 mol) of anhydrous polyoxyethylene glycol (average molecular weight 400) and 4.03 g (0.065 mol) of ethylene glycol dissolved in 193 ml of tetrahydrofuran were heated to 650C on a waterbath whilst stirring and introducing dry nitrogen gas into the reaction mixture. To the obtained solution 37.2gsolution (0.15 mol) of 4,4'-diisocyanato-diphenylmethane and 11.1g (0.05 mol) of isophor on diisocynate dissolved in 120 ml of tetrahydrofuran kept at 250C were added gradually over a period of 1 h. The addition reaction proceeded exothermally within the refluxing tetrahydrofuran. After the polyisocyanateswere added the elastomer solution was diluted with 20 ml tetrahydrofuran.The temperature of the mixture was kept for 5 h at 66 C.

Thereupon the elastomer solution was cooled down to 350C and still further diluted with tetrahydro furan up to a final volume of 531 ml.

The diluted solution contained 20 g of polymer per 100ml.

At 250C the viscosity of a 20% solution in tetrahy drofuran was 62.7 mPa.s.

By infrared analysis no free isocyanate groups could be detected.

Preparation of polyurethane elastomer E3 In a reaction flask of 500 ml provided with a thermometer, a nitrogen gas inlet, a stirrer and a dropping funnel 87.5 g of tolylene diisocyanate and 0.376 g of tin octoate were mixed at 200C.

To the stirred solution, cooled by means of a methanol/waterbath to OOC, 157.5 g of anhydrous polyoxyethylene glycol (average molecularweight 600) dissolved in 146 ml of ethyl acetate were added dropwise. The reaction proceeded exothermally but by cooling and gradual addition of said glycol the temperature was kept between 20 and 46 C.

After a reaction time of 7 h the polymer solution was diluted with tetrahydrofuran up to a total volume of ml.

Thethus obtained 20% solution was used as such to coat the sealing layer of protective material Vl.

The structure of the obtained elastomer was confirmed by infrared analysis to be:

about 8-9.

Preparation of polyurethane elastomer E4 In a reaction flask of 250 ml provided with a nitrogen gas inlet, stirrer, thermometer, dropping funnel and reflux condenser having on top a calcium chloride drying tube 37.2 g (0.15 mol) of 4,4'diisocyanato diphenylmethane, 11.1 g (0.05 mol) of isophoron-diisocyanate together with 0.198 g oftin octoate were introduced and mixed at room temperaturewhilst dry nitrogen was introduced to expel the air. to said mixture 27.0 g (0.0675 mol) of anhydrous polyoxyethylene glycol (average molecular weight 400) and 2,015 g (0.0325 mol) of ethylene glycol in 25 ml oftetrahydrofuran were added over a period of 1 h.

The reaction proceeded exothermally but by means of a waterbath the temperature ofthe reaction mixture was kept at 35 C. Stirring was continued for 4 h while keeping the temperature ofthe reaction mixture between 35 and 40 C. Thereupon the solution was diluted with tetrahydrofuran to obtain a total volume of 386 ml. The elastomer still contained a substantial amount of free isocyanategroups.

Preparation ofpolyisocyanate precursor P7 In a reaction flask of 250 ml provided with a nitrogen gas inlet, dropping funnel, thermometer, stirrerand reflux-condenserhavingatthetopa CaCl2-drying tube 16.6 g of anhydrous phenol were dissolved at room temperature in 60 ml of chloroform containing 0.49 g of 1 ,4-diazabicyclo [ 2.2.2 ] octane (triethylene diamine).

To said solution at room temperature (20 C) 51.5 g of a 75% solution in a 1/1 by volume mixture of ethyl glycol acetate/xylene of the addition of 1 mol of trimethylolpropane and 3 mol of tolylene diisocyanate were added over 1 h. The reaction was moderately exothermic and afterthe addition of said solution the temperature reached 500C. At that moment 20 ml of chloroform were added andthetemperature raised by heating up to 60 C. The solution was kept at that temperaturefor 19 hand thereupon cooled down to room temperature and poured into 1.51 of toluene.

After separating and drying under reduced pressure 58.3 g of a white powderwere obtained. By infra-red analysis no free isocyanate groups could be detected anymore.

Preparation of polyisocyanate precursor P2 In a three-necked reaction flask of 250 ml provided with a stirrer, thermometer, nitrogen gas inlet and reflux condenser having at the top a CaCl2-drying tube 11.5 g (0.1575 mol) of acetoxime dissolved in 30 ml of anhydrous chloroform and 0.33 g of 1 ,4-diaza- bicyclo [ 2.2.2ioctane were introduced.

At room temperature (200C) a mixture of 60 ml of anhydrous chloroform and 51.5 g of a 75% solution in a 1/1 by volume mixture of ethyl glycol acetate/xylene oftheaddition product of 1 mol oftrimethylolpropane and 3 mol oftolylene diisocyanate were added overa periodof45 minutes while dry nitrogen gas was introduced to keep the air out of the reaction flask. The temperature of the reaction medium reached 50 C.

Atthe end of the addition of said mixture in the dropping funnel 20 ml of anhydrous chloroform were introduced, shaked and added to the reaction mixture.

The reaction mixture was kept stirring for 6 h at 60 C by heating the reaction flask without discontinuing the dry nitrogen gas introduction.

Thereupon the reaction mixture was cooled down to room temperature and poured into 1.51 of toluene.

The precursor compound separated as a white powder. Yield after drying under reduced pressure: 49.6 g. By infrared analysis no free isocyanate groups could be detected anymore.

Preparation of polyurethane elastomer P3 In a three-necked reaction flask of 250 ml provided with a stirrer, thermometer, nitrogen gas inlet and reflux condenser having atthetopaCaClrdrying tube 8.36 g of anhydrous ethanol, 30 ml I of ofchloroform and 0.24 g of 1 ,4-diaza-bicyclo [ 2.2.2 ] octane were introduced consecutively.

At room temperature (20 C) a mixture of 60 ml of chloroform and 53.0 ml of a 75% solution in a 1/1 by volume mixture of ethyl glycol acetate/xylene of the addition product of 1 mol oftrimethylolpropane and 3 mol of tolylene diisocyanate were added over a period of 45 minutes while dry nitrogen gas was introduced to keep the air out ofthe reaction flask.

The reaction proceeded moderately exothermally whereby the temperature raised to 36 C.

Thereupon the reaction mixture was heated to reach 600C and at that temperature the reaction mixture was keptfor20 h.

The non-viscous solution was cooled down to room temperature and poured into 1.51 oftoluene.

After separating and drying under reduced pressure,the precipitate, 50.15 g ofthe precursor P3 wherein the isocyanate groups were blocked by reaction with ethanol was obtained.

Preparation of polyisocyanate precursor P4 In a reaction flask of 500 ml provided with a nitrogen gas inlet, thermometer, dropping funnel, stirrer and reflux condenser having at the top a CaClrdrying tube 73.0 g of acetoxime dissolved in 230 ml of chloroform and 2.16 g oftriethylene diaminewere introduced. The obtained solution was heated on an oilbath kept at 560C and slowly 84.0 g of hexamethylene diisocyanate was added dropwise.

The exothermic reaction proceeded of 1 h meanwhile maintaining thetemperature of the reaction mixture between 56 and 68 C.

Afterthe addition ofthe hexamethylene diisocyanate the reaction mixture was kept stirring for 1/2 h.

Thereupon the precursorwas separated by pouring the reaction mixture cooled down to 20 C into 3.61 of n-hexane in which it precipitated.

Aftersuctionfiltering and drying under reduced pressure at 200C 153.6 g ofthe polyisocyanate precursor P4was obtained.

Preparation of polyisocyanate P5 P5 is prepared by addition of 1 mol of trimethylolpropane to 3 mol oftolylenediisocyanate. It is available on the market underthetrade name DESMODUR-L of Farbenfabriken BayerA.G. Lever kusen-W.Germanyasa75% byweightsolutionin a 1/1 byvolume mixture of ethyl glycol acetate and xylene and has the following structural formula:

Claims (21)

1. A coating composition suitable for applying as a sealing coating to a hydrophilic colloid, comprising a mixture of at least one polyurethane resin and at least one polyisocyanate precursor compound, characterised in that 1) the said polyurethane resin is obtained bythe reaction of one or more polyisocyanates and one or more polyols in a proportion suchthat substantially no free isocyanate groups are present in the resulting polyurethane, and 2) the said polyisocyanate precursor compound is the addition product of a polyisocyanate with at least one compound selected from the group acetone-oxime, butanone-oxime, phenol, monochlorophenol, a di(C1-C2) alkylmalonate, the said mixture consisting offrom 60% to 95% byweight ofthe said polyurethane resin and from 5% to 40% by weight of the said polyisocyanate precursorcompound(s).

2. A composition according to claim 1, wherein the said polyurethane resin is obtained by the reaction of the said polyisocyanate(s) and the said polyol(s) in proportions such that the ratio of the totals of groupsand-NCOgroupsinthe reactants isin the range 1.05:1 to 0.95:1.

3. A composition according to claim 2, wherein the said ratio is 1 :1.

4. A composition according to any of claims 1 to 3, wherein the polyol(s) used forthe preparation of the said polyurethane resin is an aliphatic or cycloaliphatic diol, or polyoxyalkylenediol, or a mixture thereof.

5. A composition according to any of claims 1 to 4, wherein the polyisocyanate(s) used forthe preparation ofthesaid polyurethane resin is an aliphatic, cycloaliphatic or aromatic di-isocyanate, ora mixture of two or more thereof.

6. A composition according to claim 5, wherein the said di-isocyanate is a mixture of 4,4'-diisocyanato-diphenylmethane and isophoron diisocyanate.

7. A composition according to claim 6, wherein the said polyurethane resin is the reaction product of a mixture of diols and a mixture of diisocyanates in equimolar amounts, the said mixture of diols consists of 40-30 mol % of ethylene glycol and 60-70 mol % of a polyoxyethylene glycol [ HO~(CH2~CH=O)n~ OH] wherein n is 8 or9, and said mixture of diisocyanates consists of 25/75 to 50/50 molar mixture of isophoron diisocyanate and 4,4'-diisocyanatodiphenylmethane.

8. A composition according to claim 7, wherein the said polyurethane resin has a hydroxyl value not higherthan 56, the said hydroxyl value being the number of mg of KOH required to neutralize the acetic acid capable of actylating 1 g of the said resin involved.

9. Acomposition according to claim 7,wherein the said polyurethane resin has a molecularweight such thatthe viscosity of a 20% by weight solution thereof in tetrahydrofuran at250C is from 10 to 90 m.Pa.s.

10. Acompositionaccordingtoanyofclaimsl to 9, wherein the said polyisocyanate precursor(s) are according to one ofthefollowing structures:

wherein: R stems from a low molecularweight ( < 2000) aliphatic polyurethane.

11. A composition according to any of claims 1 to 10, in the form of a sealing coating applied to a web or sheet material serving as a support.

12. A composition according to claim 11, wherein the support is a hydrophobic transparent resin support.

13. A composition according to any of claims 1 to 12, wherein the sealing coating has a thickness in the range of 5 to 100,am .

14. A processforforming a laminar article in which a web or sheet material having an outer hydrophilic colloid layer is contacted with a sealing coating applied on a web or sheet Support, which coating has a composition comprising at least one polyurethane resin and at least one polyisocyanate precursorcompound and thereafter heated to free polyisocyanate in the coating composition whereby the coating becomes sealed to the said colloid layer by a cross-linking reaction with the hydrophilic colloid, characterized in that the said sealing coating has a composition according to any of claims 1 to 13.

15. A process forforming a laminar article according to claim 14, characterized in that said article is formed by placing said web or sheet material in superposition with the supported sealing coating and then to pass the superposed elements through the nip of a pair of hot pressure rollers, heating the elements in thetemperature range of 100 to 1 300C for a period of time lasting not longerthan 30 s.

16. A process for forming a laminar article according to claim 14, characterized in that said article is formed by passing the supported sealing coating in contact with the sheet or web material having a supported gelatin containing layer to be protected through the nip of a pair of heated (upto 120 C) pressure exerting rollers and secondly by postheating the laminate for a period of 2 min in the range of 100 to 1 100C,the applied pressure by the rollers being in the range of 5 to 0.1 kg/sq.cm.

17. Aprocessforformingalaminararticleaccord- ing to any of claims 14to 16, characterized in that the laminararticle is produced by sealing said supported sealing coating to a photogaph containing a layer including a silver image and/or colour image in a proteinaceous hydrophilic colloid binder.

18. A processforforming a laminar article according to any of claims 14to 17, characterized in that subbed polyester or polyvinylchloride support is first coated with a sealing coating as defined in any of claims 1 to 10 and to said sealing coating a developing nuclei containing layer is applied incorporating these nuclei in gelatin as binding agent, the thus obtained diffusion transfer reversal (DTR-) image receiving material is then used to produce thereon a silver image which isthen protected against contact with the environment and against forgery by laminating thereto a supported sealing composition according to any of claims 1 to 13.

19. A process forforming a laminar article according to any of claims 14to 17, characterized in that a recording material comprising visually detectable information and containing one or two outer hyd rophiliccolloid layers with reactive hydrogen atoms is encased between two supported sealing coatings according to any of claims 1 to 13, one supported sealing coating forming a transparent front wall member and the other a transparent or opaque rear wall member of the laminar article, the front and back members being somewhat larger in dimensions than the material comprising the information, so that by heat and pressure lamination of the supported sealing coatings with the encased material a peripheral area of the same chemical crosslinked structure of superposed sealing coating edges is formed.

20. Aprocessforforming a laminararticleaccording to any of claims 14to 19, characterized in that a recording material containing a pattern-wise applied image layer is sealed to said supported sealing coating which coating is applied over an area substantially the same or different from the area of said image layer.

21. A process for protecting a hydrophilic colloid layer by applying a solution ofthe polyurethane resin 1) and ofthe polyisocyanate precursor2) according to any of the claims 1 to 10 onto the hydrophilic layer to be protected, whereupon the solvent(s) of the solution are removed by evaporation and the applied coating composition is heated to effectthe sealing to said hydrophilic colloid layer by crosslinking reaction with freed polyisocyanate.